1. Field of the Invention
This invention relates to vertical cavity surface emitting lasers. The invention relates specifically to longer wavelength VCSELs such as 1.3 micrometer, or micron, (xcexcm) wavelengths which can be made with ordinary MOCVD equipment or MBE equipment. In general it relates to obtaining light emission at wavelengths not normally obtainable with a given material system because of lattice mismatch.
2. Description of the Related Art
Vertical cavity surface emitting lasers (VCSEL) made with GaAs are known in the art which emit light in the 850 nanometer range. Because the quantum well for the short wavelength 850 nanometer VCSELs is made from GaAs (the same material as the substrate) the various epitaxially deposited layers, whose thickness is related to wavelength, are able to maintain the minimal mechanical strain without mechanical relaxation. However, if one were to use InGaAs in the active region at the larger 1.3 micron wavelengths, the lattice mismatch is so large the layers would tend to relax their strains and suffer dislocations, slip lines or island growth which would interfere with proper lasing.
In order to go to the proper bandgap for a 1.3 xcexcm wavelength VCSEL one must use InGaAs or GaAsSb or some combination thereof instead of GaAs in the active layer. However, indiumgalliumarsenide and galliumaresenideantimonide are not the same lattice constant as GaAs at the compositions useful for 1.3 micron lasers. This makes it very difficult to build a proper quantum well structure.
It is therefore very desirable to come up with a quantum well (i.e. the active layer and the barrier layers surrounding it) which makes use of common GaAs, InGaAs or GaAsSb materials in construction of the 1.3 micron wavelength VCSEL.
The present invention extends the use of nonlattice matched quantum wells by extending the composition range over which they are mechanically stable. This is done by introducing thin regions, or mechanical stabilizers in the quantum well region, with the same lattice constant as the substrate while using thin layers of a semiconductor alloy of a different lattice constant in the quantum well structure. Alternatively, the lattice constant of the mechanical stabilizers may be nearly, eg. about xc2x12%, the same as that of the substrate, or mismatched in the opposite direction of the remainder of the quantum well material. The mechanical stabilizers are thin enough that their effect on the quantum well energy levels is small enough to be conveniently compensated for by modifying the composition, i.e. the indium to gallium ratio of the InGaAs layers or the arsenic to antimony ratio of GaAsSb or a combination of the above in InGaAsSb. A series of mechanical stabilizers is created within the quantum well structure. The effective quantum well energy level is that from the whole series of quantum well structures with mechanical stabilization layers therein. The effective quantum well energy level is modified only slightly by the presence of the mechanical stabilizers.
The mechanical stability is guaranteed by keeping the strained quantum well material between the stabilizers about or below the critical thickness as defined by Matthews and Blakeslee for nonlattice matched crystal growth. See for example p. 374 of xe2x80x98Quantum Well Lasers,xe2x80x99 Peter Zory, Academic Press 1993 for an interpretation of different critical thickness models including Matthews and Blakeslee. The mechanical stabilizers are unstrained since they are the same lattice constant as the substrate. The present invention may be generally used, but specifically applies to GaAs substrates; InGaAs, GaAsSb, or InGaAsSb quantum wells and GaAs mechanical stabilizers, or combinations thereof.
With the use of the mechanical stabilizers of the present invention active layer structures of the VCSEL may be built from common InGaAs or GaAsSb and GaAs materials used with ordinary MOCVD deposition equipment at layer thicknesses suitable for 1.3 micron wavelength emission without relaxation of mechanical strain; leading to reliable lasing in this wavelength with the use of common deposition methods and materials.